Photoluminescence and raman studies of high quality CdTe:I Epilayers

Low-temperature photoluminescence (PL) studies of iodine-doped CdTe epilayers have been performed. A compensating acceptor center which gives rise to deep-level PL emission at 1.491 eV is identified. From selective excitation PL studies, we assign this 1.491 eV line to the recombination of an associate donor-acceptor close pair, consisting of nearest neighbor substitutional sodium and iodine atoms (Na_Cd-I_Te). This neutral defect complex has a localized mode of 36.5 meV, which is much larger than the bulk CdTe lattice mode of 21.3 meV. The electronic energy level associated with this defect is 115 meV below the conduction band. Also, we use a combination of selective excitation PL and Raman spectroscopies to determine the ionization energy of the isolated shallow iodine donor (I_Te) in CdTe. We find that the donor binding energy of this anion-site hydrogenic donor is 15.0 (±0.2) meV.     

Defect Engineering of Grain Boundaries in Lead‐Free Halide Double Perovskites for Better Optoelectronic Performance

Halide double perovskites (HDPs) are promising lead‐free perovskites for various optoelectronic applications. However, the device performances of HDPs are far below the optimized values, which open a critical question regarding the origin of low performance in these HDPs. In this article, using first‐principles calculations, it is found that some types of grain boundaries (GBs) are easy to form in polycrystalline HDPs. Importantly, the existence of low‐energy Σ5(310) GBs can induce harmful deep‐level defect states within the bandgaps of type‐I (e.g., Cs₂AgInCl₆) and type‐II (e.g., Cs₂AgBiCl₆) HDPs, which may dramatically reduce the device performances. Interestingly, it is found that the formation of some intrinsic defects and defect complexes could effectively eliminate these deep‐levels in type‐II and type‐I HDPs, respectively. Under some exactly predesigned growth conditions identified by utilizing thousands of chemicals through a potential screening process, these defects or defect complexes can spontaneously incorporate into the GB cores, meanwhile the harmful deep‐level defects in the bulk can also be effectively eliminated. In addition, the self‐passivated GBs could generate band bending, which may be beneficial for charge separation. The understanding of GB formation as well as the self‐passivation mechanism in HDPs can provide a new viewpoint and guidance for designing polycrystalline perovskites with improved optoelectronic performance.     

Crystal defects and interdiffusion behavior of Hg1−xCdxTe/(100)CdTe epitaxial layers grown by chemical vapor transport

Crystal defects of chemical vapor transport grown Hg_1−xCd_xTe on (100) CdTe structures have been investigated using chemical etching, wavelength-dispersive spectroscopy, x-ray rocking curve, and scanning electron microscopy methods The results indicate that the origin and spatial distribution of the misfit dislocations can be attributed to both the lattice parameter misfit and the inevitable interdiffusion occurring between the substrate and the epitaxial layer. It is proposed that the interdiffusion of Hg along the [100] direction is enhanced by dislocation channels and other defect cores along or near this direction owing to defects on the initial surface of the CdTe substrate. The results indicate that the subgrain boundaries in Hg_1−xCd_xTe are caused by slight misorientation of the lattices and polygonization of the defects during epitaxial layer growth, and by the propagation of the subgrain boundaries existing in the CdTe substrate.     

Optical and Structural Properties of CdTe Grown by Molecular Beam Epitaxy at Low Temperature for Resonant-Cavity-Enhanced HgCdTe Detectors

Investigation into resonant-cavity-enhanced (RCE) HgCdTe detectors has revealed a discrepancy in the refractive index of the CdTe layers grown by molecular beam epitaxy (MBE) for the detectors, compared with the reported value for crystalline CdTe. The refractive index of the CdTe grown for RCE detectors was measured using ellipsometry and matches that of CdTe with an inclusion of approximately 10% voids. X-ray measurements confirm that the sample is crystalline and strained to match the lattice spacing of the underlying Hg_(1−x)Cd_(x)Te, while electron diffraction patterns observed during growth indicate that the CdTe layers exhibit some three-dimensional structure. Secondary ion mass spectroscopy results further indicate that there is enhanced interdiffusion at the interface between Hg_(1−x)Cd_(x)Te and CdTe when the Hg_(1−x)Cd_(x)Te is grown on CdTe, suggesting that the defects are nucleated within the CdTe layers.     

Uniformity of deep levels in semi-insulating InP obtained by multiple-step wafer annealing

The uniformity of deep levels in semi-insulating InP wafers, which have been obtained by multiple-step wafer annealing under phosphorus vapor pressure, was studied using the thermally stimulated current (TSC) and photoluminescence (PL) methods. Only three traps related to Fe, T₀ (ionization energy E_i=0.19 eV), T₁ (0.25 eV), and T₂ (0.33 eV), probably forming complex defects, were observed in the wafer and they exhibited a relatively uniform distribution. PL spectra relating to phosphorus vacancies observed in some regions of the wafer are correlated with a small TSC signal having an ionization energy of 0.43 eV.     

Concentrations of native and gold defects in HgCdTe from first principles calculations

We have studied the defect formation energies of the various native (vacancies, interstitials, and antisites) and Au defects in Hg_1−xCd_xTe using density functional-based total energy calculations with ultrasoft pseudo-potentials. These studies are important for infrared (IR) detection technology where the device performance can be severely degraded because of defects. To calculate formation energies, we modeled the neutral and charged defects using supercells containing 64 atoms. From the formation energies, we have determined the defect concentrations as a function of stoichiometry and temperature. We find the prevalent neutral defects to be Au at the Hg site (Au_Hg ), Hg vacancies (V_Hg ), and Te antisites (Te_Hg ). We have also explicitly studied charged defects and have found Te _Hg ²⁺ , Au _Hg ¹⁻ , V _Hg ¹⁻ , V _Hg ²⁻ , and V _Te ²⁺ to have low formation energies. We have identified Au_Hg to be the prevalent Au defect, having concentrations several orders of magnitude greater than the other Au defects. We find that the charge state of V_Hg is primarily (1−) or (2−) depending on the electronic chemical potential.     

Substituent effect of fluorine ligand on spectroscopic properties of Pt(N^C^N)Cl complexes,a theoretical study

Time-Dependent Density Functional Theory (TDDFT) method was used to investigate the substituent effect of fluorine ligand on geometrical structures, electronic properties, electroluminescent properties, absorption and emission spectra of six tridentate cyclometalated Pt(II) complexes. M062X hybrid functional was proved to be suitable for calculating the lowest triplet excited state (T₁) characters in TDDFT calculations. The energies of d–d transitions both in absorption and emission were larger than HOMO–LUMO energy gaps, so d–d transitions did not easily occur. With the introduction of fluorine ligand, the energy levels did not show regularity changes, while the IP (ionization potentials) values and EA (electron affinities) values increased correspondingly. The phosphorescence emissions of the complexes were all assigned as ³ILCT mixed with ³MLCT. In addition, one dimeric form of cyclometalated Pt(II) complexes have also been investigated.     

MBE HgCdTe on Alternative Substrates for FPA Applications

Results of first-principles calculations and experiments focusing on molecular beam epitaxy (MBE) growth of HgCdTe on the alternative substrates of GaAs and Si are described. The As passivation on (2 × 1) reconstructed (211) Si and its effects on the surface polarity of ZnTe or CdTe were clarified by examining the bonding configurations of As. The quality of HgCdTe grown on Si was confirmed to be similar to that grown on GaAs. Typical surface defects in HgCdTe and CdTe were classified. Good results for uniformities of full width at half maximum (FWHM) values of x-ray rocking curves, surface defects, and x values of Hg_1−x Cd _x Te were obtained by refining the demanding parameters and possible tradeoffs. The sticking coefficient of As₄ for MBE HgCdTe was determined. The effects of Hg-assisted annealing for As activation were investigated experimentally and theoretically by examining the difference of the formation energy of As_Hg and As_Te. Results of focal-plane arrays (FPAs) fabricated with HgCdTe grown on Si and on GaAs are discussed.     

Residual strain in cadmium telluride/gallium arsenide (001) heterostructures as a function of temperature

Optical studies of residual strain in cadmium telluride (CdTe) films grown using molecular beam epitaxy on gallium arsenide (GaAs) substrate have been performed using photoreflectance techniques. Measurements have been conducted to determine the fundamental transition energy, heavy-hole and light-hole transition energy critical-point parameters in a range of temperatures between 12 and 300 K. There are problems inherent in the fabrication of optoelectronic devices using high-quality CdTe films, due to strain effects resulting from both the lattice mismatch (CdTe: 14.6%) and the thermal expansion coefficient difference. The CdTe film exhibits compressive stress causing valence-band splitting for light and heavy holes. We have used different models to fit the obtained experimental data and, although the critical thickness for the CdTe has been surpassed, the strain due to the lattice mismatch is still significant. However, the strain due to the thermal expansion is dominant. We have found that the fundamental transition energy, E₀, is affected by the compressive strain and the characteristic values are smaller than those reported. In addition, the total strain is compressive for the full measured range, since the strain due to the lattice mismatch is one order of magnitude higher than that calculated from the thermal expansion.     

Variable temperature hall effect on p-Hg1−xCdxTe grown on CdTe and sapphire substrates by liquid phase epitaxy

Variable temperature Hall effect measurements have been made down to 9–10K on p-type Hg_1−xCd_xTe grown by liquid phase epitaxy on both CdTe and sapphire substrates. Carrier freeze-out was usually observed throughout the measured temperature range. For most samples, the hole mobility was well-behaved and exhibited a maximum at ˜ 35K. Values of acceptor ionization energy E_A and donor concentration N_D were estimated from the data, using a model assuming significant compensation, which provided a good fit to the low temperature data. In addition, values of N_D were also estimated from an analysis of the low temperature mobility using the hole effective mass as a parameter to provide reasonable agreement between the N_D values calculated from the Hall coefficient and mobility data. The measured carrier concentration is a result of close compensation between stoichiometric acceptors and donors, with N_D usually in the low-10¹⁷ cm⁻³ range. Average values of E_A for as-grown, undoped x = 0.32 layers on CdTe and sapphire substrates are 7.4 and 6.6 meV, respectively. An activation energy of 0.84 meV was determined for a Cu-doped x = 0.32 layer that was annealed in Hg vapor to reduce the number of Hg vacancies. The average E_A for undoped Hg-annealed x = 0.22 layers on CdTe substrates is 2.35 meV. Layers with x = 0.32 grown on sapphire substrates have average carrier concentrations of 2.92 (σ = 0.54) × 10¹⁶ cm⁻³, compared with 4.64 (θ = 1.26) × 10¹⁶ cm⁻³ for the same composition layers grown on CdTe substrates.     

First principle study of Nb defects in anatase (101) TiO2 surface

In this work, effects of Niobium (Nb) defects on TiO₂ surface using density functional theory (DFT) are investigated. Based on formation energy of the defects, their occurrences in two different extreme conditions, O-rich and O-poor conditions, are evaluated. Effects of Nb defects on surface and its electronic structure are studied and it is demonstrated that Nb doping widens valence band in deep energy level leaving the band gap without any change and it also lowers oxygen vacancy defect concentration due to the stronger bonding of Nb_sub defect with oxygen atoms specially bridging oxygen (most probable defect site for Oxygen vacancy). Higher density of Nb substitutional defects (Nb_sub) are examined and it is shown that higher density doping of TiO₂ surface leads to uniform distribution of defects over the anatase structure as a result of interaction of Nb defects when they are close and this fact prevents segregation of Nb atoms in Nb-doped TiO₂.     

Electrical characteristics of CdTe:Pb single crystals at high temperatures

The electrical properties of CdTe:Pb single crystals at high temperatures (400–900°C) and under controlled Cd vapor pressure (0.001–3 atm) were investigated for the first time. The temperature and baric dependences of the conductivity and Hall coefficient were measured. Low (in comparison with undoped CdTe) electron concentration indicates an increase in the number of impurity point defects related to the Pb impurity. The results obtained are explained within the Kröger theory of quasi-chemical reactions of defect formation on the assumption that lead may exist in the isolated state (Pb _Cd ⁺ ) and as a component of (Pb _Cd ⁺ V _Cd ^2? )^? associates.     

Effect of features of the technology of polycrystalline CdTe growth on the conductivity and deep level spectrum after annealing

The conductivity, morphology, and deep levels in polycrystalline CdTe are studied. Undoped p-CdTe is grown from the vapor phase by low-temperature methods of direct Cd and Te chemical reaction and CdTe vacuum sublimation at P _min. Chlorine-doped CdTe is also grown. The resistivity of the grown samples is ～105–109 Ω cm. After annealing in liquid cadmium or in cadmium vapor at ～500°C, the conductivity type changes, the free-carrier concentration in the undoped and doped samples increases to 4 × 10¹⁵ and ～2 × 10¹⁶ cm^?3, respectively. For all samples, a defect ground level of ～0.84 eV and continuous background are observed in DLTS spectra after annealing. A correlation between the primary-defect and free-carrier concentrations in undoped and doped CdTe is observed. Chlorine is a main residual impurity in the undoped samples. It is assumed that the defect is a complex including chlorine and observed structural defects in CdTe.     

Point defects in Cd1−xZnxTe: A correlated photoluminescence and EPR study

Cd_1−xZn_xTe (CZT) is an emerging material for room-temperature x-ray and gamma-ray detectors. The identification and control of point defects and charge compensators in the bulk material are currently important issues. We have used photoluminescence (PL), photoluminescence excitation, and electron paramagnetic resonance (EPR) to characterize point defects in a series of bulk CZT crystals grown by the high-pressure Bridgman technique. Luminescence peaks due to shallow donors, shallow acceptors, and deeper levels, such as V_Cd-D_Cd complexes (D = shallow donor), were monitored. This was followed by a detailed study of photo-induced EPR, using a tunable Ti:sapphire laser. There were no EPR signals in the “light off” condition; however, during illumination, an isotropic EPR spectrum due to neutral donors could be observed. The dependence of the donor g value on zinc molar fraction, x, has been determined for the range 0.07<x<0.14. We show that PL and EPR can be combined to give increased quantitative defect analysis in CZT. With resonant tuning of the Ti:sapphire laser, we detected donor concentrations as low as mid-10¹⁴cm⁻³ in detector-grade CZT crystals.     

<Emphasis Type="Italic">Ab Initio</Emphasis> Studies of Hydrogen Defects in CdTe

Using first-principles calculations based on density functional theory, we have investigated the nature of H defects in CdTe. The formation energy calculations indicate that the ground state position of the H inside the CdTe lattice depends on charge state: the lowest energy position for H⁰ and H⁺ is at the bond center site, while H⁻ prefers the tetrahedral interstitial site with Cd nearest neighbors (T_Cd). We find that H in CdTe acts as an amphoteric impurity. In p-type samples, H is in a positive charge state, acting as a donor to neutralize the free holes in the valence band, and in n-type samples H acquires an electron, compensating the donors in the sample.     

The impact of MOCVD growth ambient on carrier transport,defects, and performance of CdTe/CdS heterojunction solar cells

CdTe solar cells were fabricated by depositing CdTe films on CdS/SnO₂/glass substrates in various metalorganic chemical vapor deposition growth ambient with varying Te/Cd mole ratio in the range of 0.02 to 15. The short-circuit current density (J_sc) showed a minimum at a Te/Cd ratio of 0.1 and increased on both sides of this minimum. The open-circuit voltage (V_oc) was found to be the highest for the Te-rich growth ambient (Te/Cd∼6)and was appreciably lower (600 mV as opposed to 720 mV) for the stoichiometric and the Cd-rich growth conditions. This pattern resulted in highest cell efficiency (12%) on Te-rich CdTe films. Auger electron spectroscopy revealed a high degree of atomic interdiffusion at the CdS/CdTe interface when the CdTe films were grown in the Te-rich conditions. It was found that the current transport in the cells grown in the Cd-rich ambient was controlled by the tunneling/interface recombination mechanism, but the depletion region recombination became dominant in the Te-rich cells. These observations suggest that the enhanced interdiffusion reduces interface states due to stress reduction or to the gradual transition from CdS to CdTe. The hypothesis of reduced defect density in the CdTe cells grown in the Te-rich conditions is further supported by the high effective lifetime, measured by time-resolved photoluminescence, and the reduced sensitivity of quantum efficiency to forward/light bias.     

Relevance of Thermal Mismatch in Large-Area Composite Substrates for HgCdTe Heteroepitaxy

It is well known that the large lattice mismatch (>14%) associated with CdTe/Si, CdTe/Ge, and CdTe/GaAs composite substrates, is a great contributor to large dislocation densities and other defects that limit the performance of HgCdTe-based infrared detectors. Though thermal expansion mismatch is another possible contributor to material defects, little work has been done towards documenting and understanding its effects in these systems. Here, we perform studies to determine the relative contributions of lattice and thermal mismatch to CdTe film characteristics, including dislocation density and residual stress. Unannealed and thermally cycled films are characterized using x-ray diffraction, defect decoration, and Nomarski and transmission electron microscopy. For CdTe/Si, the residual stress is consistently observed to be tensile, while for CdTe/Ge and CdTe/GaAs, a compressive residual film stress is measured. We show based on theoretically predicted stress levels that the experimental measurements imply the dominance of thermal mismatch in the residual stress characteristics.     

IBIC characterization of charge transport in CdTe:Cl

Studies of charge transport uniformity in bulk CdTe:Cl have been carried out using ion-beam-induced charge (IBIC) imaging. High resolution maps of charge collection efficiency, mobility-lifetime product (μτ), and drift mobility (μ) were measured using a scanning microbeam of 2 MeV protons focused to a beam diameter of ∼3 μm. Excellent charge transport uniformity was observed in single crystal CdTe:Cl, with electron μτ values of up to 5 × 10⁻³ cm²/V s. The presence of extended defects such as tellurium inclusions was also studied using IBIC, and their influence on the charge transport performance of CdTe detector structures is discussed. The text was submitted by the authors in English.     

Improved reliability of large-sized a-Si thin-film-transistor by back channel treatment in H2

A hydrogen plasma treatment on the back-channel region of large-sized amorphous silicon thin film transistor (a-Si TFT) with high RF power and optimal process time of 20 s is proposed in this work to effectively reduce off current (I_off) and threshold voltage (V_th) shift under high and low electrical-field stresses. The channel width (W) of large-sized a-Si TFT is ranged from 1000 to 10,000 μm, which are comparable to the realistic TFTs used in the gate driver on array (GOA) of display. It is experimentally found that the mechanism of V_th shift (ΔV_th) after high electrical stress is dominated by the defect generation in a-Si layer rather than charge trapping in the gate insulator (GI) layer, which is different from the observation in previous literatures. It could be due to the effects of back-channel treatment (BCT). In addition, after low electrical stresses, the mechanism of ΔV_th is dominated by defect generation in a-Si layer, which is consistent with previous reports.     

Reaction chemistry and resulting surface structure of HgCdTe etched in CH4/H2 and H2 ECR plasmas

We report on several new aspects of etching of Hg_1−xCd_xTe (x = 0.22), HgTe, and CdTe in CH₄/H₂/Ar plasmas generated by an electron cyclotron resonance plasma source. Using a residual gas analyzer, we have identified elemental Hg, TeH₂, Te(CH₃)₂, and Cd(CH₃)₂ as the primary reaction products escaping from a HgCdTe surface during the plasma exposure. We have also demonstrated that a bias is not needed to etch HgCdTe at moderate temperatures (30-40°C), as previously suggested by other researchers. We have also developed a technique that avoids the formation of hydrocarbon polymer films on a HgCdTe sample during etching. Moreover, we have examined by x-ray photoelectron spectroscopy analysis and ellipsometry the surface condition of HgCdTe resulting from etching with this technique at zero bias. After exposure to the CH₄/H₂Ar plasma (or to a H₂/Ar plasma only), the HgCdTe samples exhibited a depletion of the HgTe component in the near surface region (increase of the x-value). The depletion covered a range from virtually x = 1 after H₂/Ar (10:2 in sccm) etching to values 0.4 < x < 0.5 after CH₄/H₂Ar (7:7:2 in seem) etching. Exposures to the plasmas were found to result in surface roughening of HgCdTe, however, plasmas rich in H₂ were observed to cause significantly rougher surfaces than plasmas with small H₂/CH₄ ratios. This difference in the resulting surface condition is attributed solely to chemical effects since the respective ion energies are considered to be below the damage threshold for HgCdTe in both cases. We also investigated the etching of HgTe and CdTe single crystals. The etch rate of HgTe was found to be over one order of magnitude higher than that of CdTe under similar conditions. This large difference in etch rates is assumed to be responsible for the observed preferential etching of the HgTe component indicated by the HgTe depletion of the HgCdTe surface region.